The present invention relates to a grease composition which is to be used with being packed in a rolling bearing of a spindle motor for an information apparatus or the like.
Conventionally, a grease composition in which an ester is used as the base oil and lithium soap is used as a thickener is packed in many rolling bearings such as a ball bearing of a spindle motor for a recording apparatus such as a hard disk drive (HDD) of a computer.
A typical example of such a grease composition is a grease composition in which pentaerythritol and di-2-ethylhexyl sebacate are used as the base oil, and lithium stearate and lithium 12-hydroxystearate are used as a thickener. The grease composition does not contribute to improvement of the anti-fretting property of a rolling bearing, but has advantages that the vibration value of a rolling bearing is reduced, and that the acoustic life is improved.
Because of an enhanced accuracy and an increased number of revolutions which are caused by the recent increase in recording density, a rolling bearing is requested to have a higher acoustic performance. Therefore, the acoustic performance of the above-mentioned grease composition becomes insufficient to cope with such a higher acoustic performance.
Recently, a loading mechanism for a magnetic head is being shifted from the CSS (Contact Start/Stop) method to the ramp load/unload method. When external vibrations are applied to an HDD during the transportation process or the like, therefore, an HDD motor is minutely vibrated and fretting occurs in a transfer surface of a rolling bearing, whereby the noise level is often raised. In order to solve this problem, a request to a rolling bearing for the anti-fretting property is enhanced, and it is therefore difficult for the above-mentioned grease composition to satisfy the request.
The invention has been conducted in order to solve the above-discussed problems. It is an object of the invention to provide a grease composition which can improve both the acoustic performance and anti-fretting property of a rolling bearing, and a rolling bearing which is lubricated by such a grease composition.
In order to attain the object, the grease composition of the invention is a grease composition which essentially comprises a base oil and a thickener, and characterized in that the thickener consists of a mixture of a urea compound and lithium soap.
When, as in the grease composition, a mixture of a urea compound and lithium soap is blended as a thickener, the lithium soap acts synergistically with the urea compound to largely contribute to improvement of the acoustic performance of a rolling bearing, so that the acoustic performance is remarkably enhanced as compared with the case where a thickener comprising only lithium soap is blended. Furthermore, the urea compound largely contributes to improvement of the anti-fretting property of the rolling bearing. Therefore, also the anti-fretting property can be simultaneously enhanced. This fact is supported by data of examples and comparative examples which will be described later.
The rolling bearing of the invention is characterized in that the rolling bearing is lubricated by the grease composition, and hence provided with a high acoustic performance and an excellent anti-fretting property by the action of the grease composition.
Hereinafter, specific embodiments of the grease composition of the invention will be described in detail.
In the grease composition of the invention, preferably, the mixture of a urea compound and lithium soap which is to be blended as a thickener contains the urea compound in 20 to 80 wt %. When such a thickener is blended, as described above, the lithium soap acts synergistically with the urea compound to largely contribute to improvement of the acoustic performance of a rolling bearing, and the urea compound largely contributes to improvement of the anti-fretting property of the rolling bearing, with the result that a grease composition which can enhance both the acoustic performance and the anti-fretting property is obtained.
In the case where the content of the urea compound in the thickener is smaller than 20 wt %, the thickener becomes similar to that comprising only lithium soap, and it is therefore difficult to improve the anti-fretting property of a rolling bearing. By contrast, in the case where the content of the urea compound in the thickener is larger than 80 wt %, the thickener becomes similar to that comprising only a urea compound, the anti-fretting property of a rolling bearing can be improved, but the acoustic performance is lowered. In both the cases, therefore, it is difficult to satisfactorily attain the object of the invention.
As the urea compound, useful is an alkyl group- or cyclohexyl group-terminated diurea compound, i.e., alkyl group-terminated aliphatic diurea or cyclohexyl group-terminated alicyclic diurea, or aliphatic/alicyclic diurea which is a mixture of the compounds. Particularly, diurea in which, as indicated by formula 1 below, the diisocyanate component is 4,4xe2x80x2-diphenylmethane diisocyanate is preferably used. Alternatively, diurea in which the diisocyanate component is 2,4xe2x80x2-toluylene diisocyanate, phenylene diisocyanate, or the like may be preferably used. 
(where R1 and R2 are an alkyl group or a cyclohexyl group)
As the lithium soap, preferably used is lithium soap in which lithium stearate is an essential component, and the lithium stearate contains 0 to 50 wt % of lithium 12-hydroxystearate. Even when lithium stearate is singly mixed with a urea compound, it is possible to provide an excellent acoustic performance. When lithium 12-hydroxystearate is singly mixed with a urea compound, however, it is difficult to improve an acoustic performance. Therefore, it is preferable to use both lithium stearate and lithium 12-hydroxystearate as described above. In this case, preferably, the blending ratio of lithium 12-hydroxystearate is set to be equal to or smaller than that of lithium stearate as described above. When lithium 12-hydroxystearate is blended in a percentage larger than 50 wt %, the acoustic performance is insufficiently improved.
On the other hand, as the base oil which is to be blended into the grease composition of the invention, preferably used is a blended oil comprising one or both of an aliphatic dibasic acid ester and a polyol ester, and poly-xcex1-olefin. As the base oil, only such an ester may be used. When poly-xcex1-olefin which has high heat resistance is mixed with the ester, there are advantages that the life of the base oil is prolonged, and that the anti-fretting property and the acoustic performance are further improved. The mixed quantity of poly-xcex1-olefin in the base oil is preferably set to about 20 to 90 wt %.
As the aliphatic dibasic acid ester, di-2-ethylhexyl sebacate, di-2-ethylhexyl azelate, di-2-ethylhexyl dodecanoic diacid, or the like is preferably used. As the polyol ester, pentaerythritol ester, trimethylolpropane, or the like is preferably used. As the poly-xcex1-olefin, hydrogenated decene-1(C10xcex1-olefin) oligomer or the like is preferably used. Preferably, the ester and the poly-xcex1-olefin are set to have a kinetic viscosity of about 10 to 40 mm2/s (40xc2x0 C.) in order to provide a low-torque characteristic.
In the grease composition of the invention, preferably, the content of the base oil is set to 80 to 90 wt %, and that of the thickener is set to 20 to 10 wt %, so that a worked penetration of the entire composition is in a range from 250 to 300. When a grease composition having a worked penetration of this degree is packed in a ball bearing, it is possible to obtain a low-torque ball bearing.
It is a matter of course that an antioxidant, an antirust agent, and other additives are adequately blended in the grease composition of the invention.
On the other hand, the rolling bearing of the invention is a rolling bearing in which the above-mentioned grease composition is packed to lubricate the bearing. As the structure of the rolling bearing, any one of various known rolling bearing structures can be employed. Because of the action of the lubricating grease composition, the rolling bearing of the invention exerts a high acoustic performance and an excellent anti-fretting property.
Next, further specific examples of the invention will be described.